[Editor] Blockless mixins eat into blockful mixins

[netux]

I'm bad with descriptions so here is an example:

Steps to reproduce

@mixin myMixin(myVar) {
Mixin.myVar;
@contents;
}

@include myMixin(blockless);

@include myMixin(blockful) {blockful mixin content;}

renders

blockless;
blockful mixin content;

Expected behaviour

blockless;

blockful;
blockful mixin content;